1. Field of the Invention
This invention resides in the field of electrically conductive ceramics, and incorporates technologies relating to nanocrystalline materials and sintering methods for densification and property enhancement of materials.
2. Description of the Prior Art
The ability of ceramics to withstand extreme conditions of temperature, mechanical stress, and chemical exposure without failure or with at most a very low failure rate has led to the use of ceramics in high-performance applications, such as heat engines, cutting tools, wear and friction surfaces, and space vehicles. In recent years, the use of ceramics has extended into the fields of microtechnology and nanotechnology, since the high-performance characteristics of ceramics has made them attractive candidates for meeting the increasing demands of nano-scale electronics and microelectromechanical systems (MEMS).
In addition to their mechanical properties, certain ceramics are of increasing interest for their ability to conduct electricity since various kinds of electrical devices are being designed for use in environments that require temperature resistance, high strength, and chemical inertness. In the microelectronics industry, for example, ceramics are used as substitutes for silicon, as trays and wafer carriers, and as ruggedized microchip substrates. In microwave technologies, conductive ceramics are attractive for their ability to perform effectively in high-temperature environments while absorbing or shielding components from electromagnetic interference. In the automotive industry, the high-temperature, high-strength, chemically inert, and electrically conductive character of ceramics make them attractive candidates for components such as fuel injector assemblies. The need for these qualities extends to medicine as well, since a variety of medical devices, such as implants, prostheses, and surgical devices, would benefit from a combination of electrical functionality, high strength and chemical inertness. This combination of properties is also of benefit to electrodes used in electrical power supplies such as batteries and solid oxide fuel cells. A similar need exists in analytical and testing devices for materials used as chemical sensors, gas separation materials, and materials for hydrogen absorption. In the aerospace and defense industries as well, materials with these properties are needed for aircraft and aircraft engines and for thermal management materials in human spaceflight applications.
Although ceramics are traditionally known as electrical insulators, ceramics can be made conductive in various ways. In amorphous SiCN-based ceramics, one way is by adding dopants to form electrically conductive composites. This method has been investigated by Hermann, A. M., et al., J. Am. Ceram. Soc. 84, 2260–2264 (2001), and Ramakrishnan, P. A., et al., Applied Phys. Lett. 78, 3076–3078 (2001), who used boron as a dopant for Si—C—N ceramics and reported that the resulting Si—B—C—N has a conductivity of about 10 (Ω·m)−1. Another method is by annealing the ceramic above its pyrolysis temperature. Amorphous silicon-carbon-nitride ceramics that are derived from polymers can be made conductive in this manner. Haluschka, C., et al., J. Eur. Ceram. Soc. 20, 1365–1374 (2000) report that the electrical conductivity of amorphous Si—C—N ceramics at room temperature can range from 10−13 to 102 (Ω·m)−1 depending on the pyrolysis atmosphere and subsequent heat treatments, and that the electrical conductivity has a positive temperature coefficient. In general, however, the published literature on Si—C—N and Si—B—C—N indicates that the electrical conductivity of these ceramics is only moderate.
A further disadvantage of Si—C—N and Si—B—C—N ceramics is reported by Shah, S. R., et al., Acta Materialia 50, 4093–4103 (2002), who state that the polymerization of the polymeric precursor to form the amorphous material is often accompanied by the evolution of ammonia which introduces pores into the material and also hinders the crosslinking process. Ceramic samples prepared in this manner typically have a porosity of about 10%, with a pore size of the same order or magnitude as the particle size of the powder. This results in poor mechanical properties which, together with an electrical conductivity that is only moderate, limits the use of Si—C—N and Si—B—C—N ceramics in both structural and functional applications.
While amorphous Si—C—N ceramics typically have poor mechanical properties, these properties can be improved by crystallization of the amorphous Si—C—N at high-temperature to form Si3N4/SiC composites. See Bill, J., et al., Advanced Materials 7, 775–787 (1995). Unfortunately, the electrical conductivity of the crystallized product is relatively low due to the combination of the SiC which is semi-conducting and the Si3N4 which is electrically insulating. Entirely separate from Si3N4/SiC composites and Si—C—N ceramics in general are titanium carbide, titanium carbonitride, and titanium nitride, which exhibit both high electrical conductivity and excellent mechanical properties but are difficult to sinter to full density. TiCN and TiN are primarily used as thin layers that are formed by chemical or physical deposition, as reported by Patscheider, J., et al., Plasma Chemistry and Plasma Processing 16, 341–363 (1996), and Veprek, S., et al., Surf. Coat. Technol. 109, 138–147 (1998). TiCxN1-x is also used as an additive to Si3N4 ceramics for purposes of increasing strength and improving electrical conductivity, as reported by Duan, R.-G., J. Eur. Ceram. Soc. 22, 2527–2535 (2002); Duan, R.-G., J Eur. Ceram. Soc. 22, 1897–1904 (2002); Herrmann, M., et al., CFI-Ceramic Forum International 73, 434–445 (1996); Bogkovic, S., et al., J. Mater. Synthesis and Processing 7, 119–126 (1999); and Herrmann, M., et al., J Eur. Ceram. Soc. 12, 287–296 (1993). The procedures reported in these papers involved sintering of the materials into composites by hot pressing, and the papers demonstrate that the addition of the TiN resulted in an increase in electrical conductivity from 10−10 (Ω·m)−1 for sintered Si3N4 to 103 (Ω·m)−1 for a sintered Si3N4/TiN composite containing 30 vol % TiN. Thus, while the TiN improved the electrical conductivity, the resulting value was still relatively low.
Of further relevance to this invention is the literature on electric field-assisted sintering, which is also known as spark plasma sintering, plasma-activated sintering, and field-assisted sintering technique. This process is disclosed in the literature for use on metals and ceramics, for consolidating polymers, for joining metals, for crystal growth, and for promoting chemical reactions. The densification of alumina powder by spark plasma sintering is disclosed by Wang, S. W., et al., J. Mater. Res. 15(4)(April 2000): 982–987.
All citations appearing in this specification, including published papers, patents and Internet websites, are hereby incorporated herein by reference in their entirety for all purposes legally capable of being served thereby.